When it comes to questions about oxygen in beer, I think the one I’m asked most often is, “What is the difference between dissolved oxygen and total package oxygen (TPO)?” The main source of this confusion is that when measuring O2 in packages, the O2 in the headspace is often overlooked. If you don’t take headspace oxygen into account, then you are measuring a partial concentration, period. So let’s talk about the differences and what each one tells you.

A significant number of craft brewers have a dissolved oxygen (dO2) analyzer they use to measure the dO2 content of their beer in process. The most common point of measurement is the finished beer tank. The beer in a finishing tank will have O2 pickup from the empty vessel and from the filtration process, plus it will pickup more O2 as it goes through packaging.

Once the beer is packaged, however (assuming good packaging,) rapid O2 pickup from outside sources all but stops. So what can we tell about how much oxygen actually made it into the package? It is not a simple matter of measuring the O2 in the beer. The package must be shaken to equilibrate the oxygen in the beer and the headspace before the 02 in the beer is measured, and that number must then be used to calculate your TPO. Let’s think about what it is possible to measure and what each thing tells you.

Package dO2 –

The easiest measurement to take on packaged beer is the dO2 of a package just off the filler without shaking the beer. It is important to measure as quickly as possible, so the product does not “consume” the oxygen in the beer. (Residual or live yeast may be hungry, plus oxidation by trace metals, etc.) In some packages there is a measurable difference within five minutes and in other packages the rate of oxygen consumption takes significantly longer, sometimes hours. It is always best to measure as quickly as possible.

This unshaken package measurement represents the combination of the dO2 of the beer at the base of the filler and the oxygen pickup of the filler. Oxygen picked up at the filler can be quite variable. Most fillers run at about 25 to 50 percent deviation, but in some cases it can be up to 100 percent deviation. The best way to measure the percent deviation is to determine the dO2 at the base of the filler and then measure six to ten packages and determine the variation of each package as compared to the average of all the containers. But remember: this measurement only tells you what is in the liquid. When measuring unshaken packages, any gas in the headspace is left uncounted.

Shaken Package dO2 –

When you shake a package of beer so that the partial pressure of the oxygen in the liquid is equal to the partial pressure in the headspace, it changes the characteristics of the oxygen partitioning in the package. If most of the oxygen in the package is locked in the liquid, then shaking the container will move the O­2 from the liquid to the headspace until equilibrium is reached.

So, you have measured the dO2 and then shaken the package. Now what do you do with the data? If you really want to quantify the TPO of the package you have to take into account the headspace oxygen. To do this accurately you need to know the headspace volume and the package temperature.

Total Package Oxygen –

When using the dissolved oxygen measurement, the TPO can only be calculated from a shaken package. To do this calculation you also need to know the headspace volume, liquid volume and the package temperature. The temperature and the headspace volume are critical values and small inaccuracies can alter the results significantly, but the liquid volume may be estimated by using the average fill volume. Once you have your figures, then you can use a TPO calculator to determine the concentration from your initial DO2 measurements.

My final thought is to not skimp on how much you shake the packages. Cold containers should be shaken for five minutes and room temperature cans or bottles need about three minutes. If you’d like a copy of a TPO calculator built into an Excel spreadsheet, then please click here to request one.

When an industrial supplier sets a minimum purity for the CO2 they supply to your brewery, you need to be aware of the ramifications of that purity and whether there is any chance it will increase the dissolved oxygen concentration of your beer. CO2 specified at 99.5% or better may sound very pure, but when we do the math we find this is actually a problem. This post is specifically about carbon dioxide that is “injected” into beer. Another post will address CO2 that is “sparged” into beer.

If your CO2 has a 99.5% purity, then the impurity is 0.5%. For purchased CO2, the assumption is that the impurity is always air, so only 1/5 of the impurity – 0.1% — should be oxygen. If you were to add to your beer one Volume of CO2 with an impurity of 0.1% O2, it would increase your oxygen concentration by a whopping 1,420 ppb.

In 1985, Nick Huige and his Miller Brewing Company co-workers published a paper on this subject in the MBAA Tech Quarterly. Their findings on the affect of injecting impure CO2 are astounding and can been seen in the table below.

Amount of added CO2

Concentration of O2 impurity in CO2

0.001%

0.005%

0.02%

0.5 V/V

7 ppb

35 ppb

142 ppb

1.0 V/V

14 ppb

71 ppb

284 ppb

2.0 V/V

28 ppb

142 ppb

567 ppb

Dissolved oxygen added to the beer

So if you want to inject CO2 into your beer, you need to be mindful of the actual purity. I know a brewer whose CO2 specification from his supplier was 99.5%. Most of the time the supply was much purer >99.998%, which is excellent. But when they had an unexpected increase in their dO2 levels, the cause was eventually traced back to the CO2. What was the purity of the “problem” C02? A number that still sounds good on a cursory level – 99.97%! – but was not acceptable in the context of dissolved oxygen in the product.

Most brewers specify a minimum CO2 purity of 99.990%. This equates to an oxygen impurity of about 0.002%. If one V/V of CO2 with this oxygen content were injected into beer, the resulting increase to the beer dO2 would be about 28 ppb.

My final thought is that if you are injecting CO2 into your beer, be sure your purity specification is not too low. You don’t ever want to be in a position where you’ve been getting great CO2 but then have one “bad” batch – still within the manufacturer’s specification – adding too much oxygen to your product.

When a dissolved oxygen monitor is your only option for measuring package oxygen content, the best way to calculate the total package oxygen (TPO) is from a shaken (equilibrated) container. In my last blog post I wrote about what you can learn by measuring unshaken packages. This time we’ll focus on equilibrated packages, which are containers that have had sufficient shaking to bring the gases in the headspace and liquid to equilibrium. First, let’s define equilibrium.

The gases in the headspace and liquid of a package are in equilibrium when their partial pressure (also called percent concentration) is the same. The only way to create equilibrium after filling is to shake the package, so that the gases move from the area of higher concentration to the area of lower concentration and are finally distributed equally.

Packages can be shaken by hand, but if you’ll be shaking a lot of packages then you’ll probably want to use a rotary platform package shaker.

Packages must be shaken for different lengths of time, depending upon their temperature. This is because warm packages reach equilibrium faster than cold packages. As a general rule, warm containers need about three minutes of vigorous shaking and cold packages need about five minutes. When shaking cold packages, it is important to keep shaking the container up to the point of use and not let the package warm after the shaking is completed. If the package warms between the time it was shaken and the time it’s measured, then the oxygen partitioning in the package will have changed and you may underestimate the TPO.

What does a shaken package tell you? Since packages come off fillers with different amounts of gas in the liquid and headspace, the most practical way to calculate the total gas content is to equilibrate them and then make your dissolved oxygen measurements. If you measure the dO2 of a freshly filled package without shaking, then you’re only determining the oxygen content of the liquid, without any feedback as to whether there was sufficient fobbing of the headspace.

Knowing the TPO not only helps you determine exactly how much oxygen is trapped in the container and can react with the beer, but it also allows you to calculate the headspace contribution to the package oxygen concentration. The headspace oxygen of a package is the TPO minus the dissolved O2:

Headspace O2 = TPO – unshaken dO2

My final thought is that if you want to measure the TPO of your packages using a dissolved oxygen sensor, you must shake the packages and measure the dissolved oxygen in as short a time frame as possible. Since different beer types have different residual package O2 consumption rates, understanding your specific beer will also help you know just how quickly this needs to be completed. We’ll talk about that in a future post.

For a review of a previous post on what you need to know to measure total package oxygen, follow this link.